Chapter 13: Competition
Intra-,
interspecific; resource, interference competition
Damsel (and other) fish example
Pine forest root trenching experimentIntraspecific Competition -- the logistic growth curve
has the assumption with it that intraspecific
competition for limited resources
largely determines K.Examples:
PLANTS
Sorghastrum nutans (competing for N in soil); Medicago sativa
(Alfalfa)
Two important results can occur as
plants are grown in increasing density:
1. Low
density -- plants of same species may be larger than when grown under higher
density. WHY?2.
Self-thinning -- exhibited by many plants; the idea being that as a patch
of one species
grows, more and more of the biomass is in fewer and fewer large individuals.
WHY?Examples:
ANIMALS
Homopterans
-- aphids, planthoppers, leafhoppers. All share a feeding habit almost
unique
in the insect world -- can tap into flowing "sap" with a very flexible "beak"
At higher densities, these insects experience reduced survivorship, slower
growth and
development, resulting from reduced protein, water content and photosynthetic
capability
of host plants.
Isopods (Grosholz)
-- supplementing food APPEARED to have little effect on survivorship
(though Grosholz used what turned out to be rather low densities of isopods),
but higher
density decreased survivorship, apparently because of cannibalism. Is
there a possible
paradox here? Should Grosholz have done further experiments to clarify
food limitation?

Niches -- a niche consists of all the necessary
factors to allow a species to subsist in a particular
place and time. Another way of
looking at the word niche would be that the niche represents the
role the species plays in the
ecosystem -- all it's interactions, including what it eats and, in turn,
acting as food for other predators.
One important result of this definition is that the niche does
not exist without the species.
Although you may see the discussion of empty "niches" in various
ecosystems, technically this is
strictly an incorrect usage of the term as there IS no niche without
the species, as indicated, and there
may be no single species that can "fill" the "niche" that is
being considered empty -- instead, IF
species "fill" the niche, it could be many species that divide
up the "empty niche".Fundamental vs. Realized Niche -- realized
niche breadth necessarily narrower than fundamental
niche breadthThe Competitive Exclusion Principle --
extinction versus resource partitioning resulting from character
displacement (remember MacArthur's warbler study from Chapter 1?)Examples:
ANIMALS
the warbler
example mentioned above
the classic:
Galapagos (Darwin's) Finches -- Beak width and seed size. Different
species
feed on different sized seeds based
on beak width and length. During drought times (remember
from Chapter 11), smaller & softer
seeds eaten first, leaving larger & harder seeds. So drought
may impact the smaller species more
severely, and within species, it may be the larger, stronger
individuals that survive through the
drought. So, food and water availability largely shape the
niche of the finches (of course,
many, many organisms are largely influenced by food/water)

Interspecific Competition -- Mathematical Models.
(Lotka/Volterra models)
This involves modifying the logistic growth curve to include competitive
effects.
dN1/dt = rmax1
N1 ((K1 - N1- α12N2)/K1)
This represents growth of species 1 with affects
on numbers from competition with species 2.
dN2/dt = rmax2
N2 ((K2 - N2- α21N1)/K2)
This represents growth of species 2 with affects
on numbers from competition with species 1.α12
&α21 are
competition coefficients, where the first represents competitive effects of
species
2 on species
1, and the second represents competitive effects of species 1 on species 2.
If α12
> 1, then competitive effects of species 2 on 1 are greater than intraspecific effects.
If α12
< 1, then competitive effects of species 2 on 1 are less than intraspecific
effects.
Zero population growth will occur for
each species when:
N1 =
K1 - α12N2
(for species 1) and N2 =
K2 - α21N1 (for
species 2)
Hopefully, it is obvious that with
the added competition from another species, the ACTUAL
value for K
COULD go down for both species. Also, needless to say, if you added yet
other
competitive
species, the equation becomes more complex, but K COULD "decline" for
EACH species
involved. However, depending on competitive capabilities of the species,
one could
exclude the other, or they could partition the resources and coexist (see pg.
290)Laboratory Examples:Paramecium
species: aurelia and caudatum
Grown separately, both species quickly reached a K based on intraspecific
competition
Grown together, aurelia outcompetes caudatumTribolium
(flour) beetles: confusum and castaneum
Grown separately, the species both did approximately equally as well.
Grown together, castaneum outcompetes confusum when conditions
were warm and
humid, but confusum outcompetes castaneum when conditions were cool and drier.

Clearly, competition can have a very strong influence on shaping niches (in
the lab). What
about in
nature?Competition and NichesNatural examples:
PLANTS
Bedstraw (Galium)
species on acidic (saxatile "wins") vs. basic (sylvestre "wins") soils
Both establish healthy populations on a variety of soils when grown separatelyNatural examples:
ANIMALS
Barnacles:
Chthamalus and Balanus (mentioned before in Chap 9, pg. 203)
Competition critically important in middle intertidal, but
desiccation-resistance becomes
more important in upper intertidal, and predation becomes more important lower.
Desert
Rodents: Dipodomys (Kangaroo rats; large granivores);
Perognathus (Pocket
mice; small granivores); and Onychomys (insectivorous rodents)
very ambitious study in SE AZ with large plots and removal strategies; we'll
discuss
this example at some length (make sure to read pages 295 and 296 carefully, and
understand figs on page 296 & 297).Character Displacement -- will
expand further on our previous discussion of Geospiza
finches and
then other organisms. A definitive example of character displacement must
meet
several criteria (page 298).

Chapter 14: Exploitative Interactions -- Predation, Herbivory,
Parasitism, DiseaseTrophic levels in the ecosystem:
Plants (the producers), herbivores (first order
consumers; depending on plant consumed,
may or may not eat entire plant),
predators (second and higher level consumers; DO kill
food), parasites (feed on, but
do not usually kill, host), parasitoids (larva consumes host,
often from inside; functionally
equivalent to predators), pathogens (induce disease; for the
most part, functionally equivalent to
. . . ?). All consumers are exploitative, i.e., make their
living at the expense of others.

Complex Interactions -- There are far more
interactions than numbers of species. WHY?Pathogens/Parasites that alter
Host Behavior
Examples:
ANIMALS
Starling (Sturnus vulgaris), Isopod (Armadillidium vulgare), and a
parasite (a spiny-
headed worm [acanthocephalan], Plagiorhynchus cylindraceus). The
parasite
changes the phototactic behavior (- to +) in the isopod. WHY?
(See
Fig. 14.2).
Fungal parasites of moth caterpillars. Near maturation of the fungus
inside the host, the
caterpillar crawls to top of a plant, becomes stiff and dies. WHY?
Examples:
PLANTS
Mustard (Arabis sp.) host and a rust fungus (Puccinia monoica).
Puccinia alters life
history of infected individual, causing plant to form a pseudoflower,
with sweet sticky
hyphae that attract "pollinators" to move spermatia from one fungus to another.
Flies
are the most common and effective rust fertilizers, but bees and butterflies
also can.
Infected plants may be killed, though some survive . . . however, those that do
never
flower, so fitness of infected plants is zero.Competition/Predation/Parasitism overlap
Obviously, it will be difficult to untangle the different interactions and their
individual
effects on organisms; some experiments, however, show clear alterations in
numbers
and success due to competitive and exploitative interactions.
Examples:
Tribolium castaneum/confusum
Besides being affected by humidity/temperature, a parasite (Adelina tribollii)
also
influences the competitive balance. Both species of Tribolium a bit
cannibalistic on
eggs, but T. castaneum strongly prefers T. confusum eggs when
present (explains a
LOT about the competition experiments). The Adelina parasite
strongly affects castaneum but not confusum, largely shifting the competition in
favor of confusum.

Exploitation and Abundance -- Predator and prey
abundance intimately tied together
Already talked about several examples
(like the Canadian Lynx, Snowshoe Hare, Dwarf
Willow
example -- see below); more examples presented here in Chapter 14:Examples:
1. Bats and birds and arthropod densities on plants (in Panama).
2.
Cactoblastis cactorum and Opuntia stricta in Australia; the moth was introduced
from South America as a biocontrol
agent. Larval feeding introduces both fungal and
bacterial pathogens into the Opuntia.
Extremely effective -- reduced pops from 12,000 per
hectare to 27, and from area of
infestation of 24 million hectares to a few thousand. This same
moth, however, has turned out to be a
potential threat to North American Opuntia, as the
moth is now spreading out from
Florida north and westward.
Similarly,
rabbits overran parts of Australia as well early in the last century, but were
largely
brought under control by introduction
of the rabbit papilloma virus.
Both rabbits
and Opuntia are now in low abundance in Australia, where much lower
density
allows them to escape the moth/virus
for a while (probably similar to where they naturally occur).
3.
Foxes, Hares and Mange Mites (Sarcoptes scabiei) in Sweden; as mange
spread across
Sweden, reduction of fox populations
resulted in increased hare populations, as well as other
prey species of the fox (grouse, deer
fawns).

Dynamics -- Virtually all interactions are
dynamic; as indicated previously, parasites may reduce
host populations which may increase
populations of prey (what would happen to a predator
whose prey was affected by a
parasite/pathogen?). However, very few of these interactions
result in some sort of continuously
stable population levels for all species involved; instead, the
populations are temporally
dynamic.Example: Again, the classic example
of the Canadian Lynx/Snowshoe Hare/Dwarf Willow (and
other shrubs) shows how abundances of
each species are tied to each other, and may change
through time. An added interactive
effect we had not discussed before is that the shoots of the
Dwarf Willow (and others) that the
Snowshoe hares browse on increase their terpenoids and
phenolic resins in response to high
rates of feeding, making the plants less tolerable to hares,
effects which can persist for up to
two years.
On the
predator side of the story, there are other predators which feed on Snowshoe
Hares,
such as foxes, coyotes, goshawks,
owls, and mink/weasel. It appears as though these predators
experience some population cycling as
well. Coyote diet may be 2/3 Snowshoe hares at peak
hare density, suggesting a predatory
functional and numerical response to hare density. Both
coyote and lynx numbers increase 6 - 7 fold during high hare
densities, though the functional
responses were a bit different. Both killed more than they could eat
at the highest hare densities,
with coyotes in particular caching for later, though coyotes kill the most during hare increases
(and apparently would kill
more if they could as their predatory response showed no sign of
levelling off), while lynx killed the
more when hare pops were declining already (probably
attacking already weakened individuals).
Experimental manipulation of food abundance and
predator abundance led to expected
results (see Fig. 14.15).

Mathematical Modelling of population cycles:
(more Lotka/Volterra "stuff")
dNh/dt = rhNh
- pNhNp where "h" refers to host & "p" refers to
predators/parasites/pathogens
"p" is the RATE of predation/parasitism, which means that "pNhNp"
is the rate at which exploiters destroy
hosts in the whole population.
Similarly for the
predator/parasite/pathogen population numbers:
dNp/dt = cpNhNp
- dpNp where "c" is a conversion factor
representing rate at which exploiters
convert hosts to offspring; "d" represents death rateSEE Figs.
14.17 & 14.18. These equations tie numbers of hosts to numbers of
exploiters --
indeed, as Nh increases,
so, too, does exploitation rate (pNhNp), which, in turn,
declines Nh and,
in turn,Np.
So, increasing host and exploiter population numbers in this model lead to
significant
decrease in numbers later -- a cycle
that is repeated. Though this model has some obvious utility,
remember again, however, that NO
system in nature is so simple as to be ONE prey item with
just ONE predator or
parasite/pathogen.
Laboratory
attempts to produce cyclic fluctuations in exploiter/host populations have proved
extremely difficult, though Utida was
able to show remarkable cyclic fluctuations between prey
bean weevils (Callosobruchus
chinensis) and it's parasitoid wasp (Heterospilus prosopidis) for
up to 112 generations in very small,
simple, environmentally controlled, petri dish habitats.

Refuges -- to persist in the face of
exploitation, prey/hosts need refugesExamples:
1.
Didinium nasutum (predators) and their prey Paramecium aurelia (see
Fig. 14.19)
Gause was able to produce
oscillations, but only if there was a refuge for the Paramecium, and
an immigration source (lab cultures)
for the Didinium.
2.
Huffaker and the ballooning/crawling herbivorous mite (Eotetranychus
sexmaculatus) and its
crawling predatory mite (Typhlodromus
occidentalis). The ballooning capability of the herbivore
allowed it to get to at least
temporary refuges on other fruits ahead of the predator, and resulted in
oscillations of predator-prey
populations (see Fig. 14.20)Spatial Refuges -- typically what we think
of in terms of refuges; Opuntia has not been driven
extinct in Australia as a result of the
introduction of the moth Cactoblastis cactorum. Why? It
has spatial refuges where, at least momentarily,
it has escaped the moth. Numerical Refuges -- "safety in numbers".
Hosts/prey can overwhelm exploiters, resulting in predator
satiation.Examples: 1. The
thirteen and seventeen year periodical Cicadas
2.
Masting by plants (synchronous seed/fruit production), often after
disturbance (eg., fire)Size as a refuge -- Elephants; Pisaster
starfish and their mussel prey (Mylitus)
Though young may fall prey to
predators, many species reach an ultimate adult size that puts them
outside the
range of prey that can be handled by the predator.
This has led to "appear big"
behaviors -- cat defensive posture; ephemerellid mayfly nymph posture

Plant Mutualisms -- (some already mentioned above)Plants and Mycorrhizal Fungi -- a
400 million± year relationship
[a facultative mutualism]
Some plants grow root "chambers" for the fungi, other types form network around
roots
These fungi
help plants with water extraction; nitrogen and phosphorus/copper/zinc
sequestration;
access to more phosphorus allows
plants to grow more roots, which allow them to claim more
water. The fungi, in turn, get
carbohydrates produced by the plant. Conflict
of interest? The balance sheet is not always nice and equal.
Different mycorrhizal
species and even different strains
within the same species may not "treat" their hosts the same, and
may parasitize the host plant.
Fungi may be more likely to act parasitically in more fertile soils,
as plants release less soluble carbs in root exudates
in more fertile soils, which apparently has
selected for more aggressively carbohydrate
sequestering mycorrhizae. In Andropogon (bunch-
grass), both shoot growth and
inflorescence production are maximized when grown with associated
mycorrhizae from unfertilized soils
rather than fertilized.Bullhorn Acacias and ant (Pseudomyrmex)
symbiotes
[an obligate mutualism for ant]
Hollow thorns
for ant abodes. The ants are fast, aggressive, agile climbers, that
vigorously attack
growing vines/epiphytes or other
animals (caterpillars) that grow/eat the tree, which is already well
protected by nasty spines!
Additionally, the Acacias provide foliar nectaries and Beltian
bodies
to "keep" ants fed and on the host
tree. Acacias without ants both grow slower (leaves, shoots,
thorns, etc.) and show higher
infestation (by other insects) and lower propagation.

Conflict
of interest? The ants may be at odds with pollinators, both in
competition for nectar and
driving them away. However,
inflorescences offer scant nectar, and have an ant repellant chemical.Aspen Sunflowers (Helianthella) and
Formica (and other) ants in the temperate zone Ants are
attracted to this Rocky Mtn. sunflower because of extrafloral nectaries,
with nectar
rich in sucrose and numerous amino
acids, sufficient to keep the ants attention. During times when
the flowers are budding and just
opening, ants rigorously patrol the flowers and help deter potential
bud/seed predators, but once flowers are open,
the rays form a "shield" between the nectaries and the
open face of the flower, so the ants do
not generally interfere with pollinators.
There are
several species of ants involved here, and NONE have an obligate relationship
withHelianthella. The reason
probably has to do with, at least in some populations, late frosts which
kill the flowers (including the
nectaries) -- if ants were obligate, the ants would die at that point.
This
event actually helps the sunflower,
as it kills any and all seed predators in that year. Clearly, the
TEMPERATE climate plays a significant
role in the nature of this mutualism.

Coral Mutualisms -- a mutualism between the coral and
algae (zooxanthellae)
Coral reefs
have exceptional diversity, and among the highest productivity of any ecosystem,
even
though surrounded by the nutrient poor ocean. How?The mutualism: dinoflagellates that
are photoautotropic, and therefore productive. They live within
coral tissues, and receive nutrients
from the coral animal, while the coral receives organics from the
zooxanthellae produce during PS.
Corals induce release of nutrients from zooxanthellae when
needed by coral with chemicals that
change zooxanthellae cell memb permeability, whereby they
release large amounts of fixed
carbon. The corals also keep growth of the zooxanthellae at 1/10 to
1/100 that of individuals cultured
separately, because of higher levels of carbon fixation. Doesn't
sound like a mutualism so far, but . . .
The zooxanthellae appear to get access to higher levels of nutrients, especially
nitrogen (in
ammonia waste). Corals with
zooxanthellae partners, don't APPEAR to release ammonia, as it is
metabolized by the zooxanthellae.
Crab/Shrimp mutualism with coral:
The coral
gain protection, and the crab/shrimp mutualists get food (extra fat bodies
produced)
and shelter.

Other examples of mutualisms: Orchids and orchid bees (obligate, see
video), Yucca and Yucca Moths
(obligate), Mullerian mimicry (facultative), the honeyguide
and humans (see the "Applications" section
of chapter 15).

Evolution of Mutualism -- in general, we would
expect mutualism to evolve and persist in a population
when and where mutualistic
individuals have a higher fitness than non-mutualistic individuals. This
will happen when the benefits to the
two organisms exceeds the costs of the relationship.